The present invention relates to an atomic absorption photometer, and more particularly, to an atomic absorption photometer which carries out a correction in a background absorption using a continuous spectrum source, such as a deuterium lamp.
When atomic absorption photometers are roughly classified, there are a flame method and a furnace method. In the flame method, a sample is sprayed in a flammable gas, and by burning the mixed gas on a burner head, the sample is heated at a high temperature to be atomized. Then, a measuring beam is allowed to pass through the atomic vapor to thereby measure an absorbance. In the furnace method, a sample is held in a graphite tube (heating tube), and by heating the tube, the sample is heated at a high temperature to be atomized. Then, the measuring beam is allowed to pass through the atomic vapor to thereby measure the absorbance.
In these atomic absorption analyses, for example, in case a large amount of salts and the like are mixed in the sample, they are not completely dissociated even at the high temperature, and may cause the scattering and absorption with respect to the measuring beam from a light source. Also, in the flame method, if a flame temperature is low, an absorption by a molecular type in the flame may occur. These phenomena are called a background absorption. In case of using a light source which emits a bright line spectrum, such as a hollow cathode lamp (HCL) which is often used as the light source in general, an absorbance by the atomic absorption of the specified element is added to an absorbance by the background absorption described above. Thus, it is difficult to obtain the accurate absorbance.
Therefore, there has been conventionally conducted a correction called a deuterium lamp background correction, which is also called a continuous spectrum source system. Namely, other than the light source which emits the bright line spectrum, an absorbance with respect to a measuring beam from a light source which emits a continuous spectrum (normally, the deuterium lamp is used) is measured at the same time. In case of using the light source which emits the continuous spectrum, since a wavelength bandwidth is extremely broad, the absorbance due to the atomic absorption of the specified element is small such that it can be substantially ignored, and it can be considered that only the absorbance by the background absorption is measured. Therefore, by calculating a difference between these absorbances, an effect by the background absorption is eliminated, and only the absorbance by the specified element can be obtained. Incidentally, throughout the specification of the present invention, in case the background correction is merely referred to, it means the background correction using the deuterium lamp.
Also, as an error factor other than the background absorption, there might be an effect of an extraneous light or outside light. Namely, the flame in the flame method and the graphite tube in the furnace method emit lights by themselves, and since a sample chamber becomes hot in the atomic absorption photometer, the sample chamber is often in an open condition, so that the light from outside may be introduced into a spectroscope. Although these lights are irrelevant to the atomic absorption, in case these lights have components of wavelengths which are observed, they cause enormous errors in the measurement. Thus, it is necessary to eliminate these lights.
In order to eliminate the effect of the background correction and the outside light, in the conventional atomic absorption photometer, lights or pulse lights of the light sources are controlled so that three periods, that is, a period of light of only the hollow cathode lamp, a period of light of only the deuterium lamp, and an off period of tuning of f both lamps, are provided. Then, during the off period, a signal z due to the outside light or dark current of a photodetector itself is obtained, and by subtracting the signal z respectively from a light receiving signal h obtained at the photodetector during the period of light of only the hollow cathode lamp, and from a light receiving signal d obtained at the photodetector during the period of light of only the deuterium lamp, light receiving signals hxe2x80x2 and dxe2x80x2, in which the effect of the outside light is eliminated, are obtained. Furthermore, absorbances are respectively calculated from the light receiving signals hxe2x80x2 and dxe2x80x2, and by finding the difference between these absorbances, the background correction is carried out.
In the atomic absorption analysis, the background correction is not always carried out, and for example, in case it is known in advance that the sample does not contain substances which cause the background absorption, the background correction is not required. Also, in case a measurement object wavelength is in a wavelength region (more than 425 nm) in which an emission spectrum of the deuterium lamp is very weak, the background correction is not useful, so that it is not required. At the time of the measurement without the background correction described above, in the conventional atomic absorption photometer, there is carried out only a control such that the deuterium lamp which is unnecessary as the measuring beam is left to be turned off, and it has not been considered to positively improve a signal-to-noise ratio of the signal at the time of measurement without the background correction.
Accordingly, the present invention has been made in view of the foregoing, and an object of the invention is to provide an atomic absorption photometer, in which a signal-to-noise ratio of a light receiving signal at the time of measurement without background correction is improved, so that the absorbance can be calculated with high accuracy.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To achieve the aforementioned object, the present invention provides an atomic absorption photometer, in which light emitted from a light source is allowed to pass through an atomization section, and the light passing through the atomization section is introduced into a photodetector or first photodetector to measure an intensity thereof, so that an absorbance at the atomization section is calculated from a signal from the photodetector. The atomic absorption photometer as stated above comprises a first light source for emitting a bright line spectrum light; a second light source for emitting a continuous spectrum light; and measurement controlling means for dividing a predetermined measurement period into three periods formed of a first measurement period by a light emitted from the first light source, a second measurement period by a light emitted from the second light source, and a third measurement period in a condition that neither of emitted lights from the first and second light sources are emitted on the photodetector in case a background correction is carried out. Also, the measurement controlling means distributes the second measurement period to one or both of the first measurement period and the third measurement period in case the background correction is not carried out.
At the time of the measurement without the background correction, the measurement by the light emitted from the second light (for example, deuterium lamp) is not required. Thus, in this case, the measurement controlling means distributes the second measurement period, which is assigned to the measurement by the emitted light from the second light source in case of the measurement with the background correction, to one or both of the first measurement period by the emitted light from the first light source and the third measurement period in the condition that neither of the emitted lights from both the light sources are emitted. Therefore, at least one of the first measurement period and the third measurement period becomes longer than that at the time of the measurement with the background correction.
In the photodetector which receives the measuring beam or light, as a period of time for receiving the measuring beam becomes longer, the light receiving signals are integrated to thereby improve the signal-to-noise ratio. If the signal-to-noise ratio of the light receiving signal is improved, the accuracy of the absorbance calculated based on the light receiving signal is improved. Thus, according to the atomic absorption photometer of the invention, especially in case the measurement without the background correction is carried out, the signal-to-noise ratio of the light receiving signal is improved as compared with the conventional atomic absorption photometer, resulting in increasing the accuracy of the absorbance.
Also, in the atomic absorption photometer according to the present invention, there may be provided filtering means for reducing a radio-frequency region of the light receiving signal by the photodetector, and the cut-off frequency of the filtering means can be changed in accordance with the existence of the background correction.
The filtering means is provided for removing a high frequency noise contained in the light receiving signal, and it is preferable to set a high region cut-off frequency of the filtering means as low as possible within a range of not losing the frequency component as the object. Normally, since the cut-off frequency is determined by the repeating cycle of the first measurement period, the second measurement period, and the third measurement period, in case the second measurement period is distributed to the first and third measurement periods as described above, the cut-off frequency can be lowered. According to this structure, since the high frequency noise is further reduced in case the background correction is not carried out, the signal-to-noise ratio of the light receiving signal can be further improved.
Also, the atomic absorption photometer according to the present invention may be further provided with a second photodetector including a detection sensitivity in a long wavelength region, and an optical path switching means for selectively introducing the light passing through the atomization section into one of the photodetector and the second photodetector. In this structure, in case the background correction is not carried out, the optical path switching means switches the optical path such that the light is introduced into the second photodetector.
Here, for example, the first photodetector is a photomultiplier, and the second photodetector is a photodiode. Although the photodiode has a disadvantage in the response speed, in case the second measurement period is distributed to the first and third measurement periods as described above, the first and third measurement periods become longer, and the slowness of the response speed does not cause so much problem. In general, since the background correction is not carried out in most of the cases when the wavelength as the measurement object is in a long wavelength region outside a range of the wavelengths of the emitted light of the deuterium lamp, it is possible to sufficiently use the detector, such as photodiode, which does not have a detection sensitivity in a short wavelength region. According to this structure, since the detection of the light in the long wavelength region can be assigned to the second photodetector, the photodetector having the detection sensitivity in the short wavelength region is sufficient. Among the photomultipliers, the photomultiplier having this characteristic is considerably cheaper than a photomultiplier having a high detection sensitivity also in the long wavelength region, and the photodiode is much cheaper. Therefore, according to this structure, the cost of the atomic absorption photometer can be further reduced.